Parametric cooling of a degenerate Fermi gas in an optical trap

We demonstrate a technique for cooling a degenerate Fermi gas in a crossed-beam optical dipole trap, where high-energy atoms can be selectively removed from the trap by modulating the stiffness of the trapping potential with anharmonic trapping frequencies. We measure the dependence of the cooling effect on the frequency and amplitude of the parametric modulations. It is found that the large anharmonicity along the axial trapping potential allows one to generate a degenerate Fermi gas with anisotropic energy distribution, in which the cloud energy in the axial direction can be reduced to the ground state value.

Figure 1

The local radial trap frequency of a crossed-beam optical trap. The $x$-axis trap frequency ${\omega }_x(x,0,z)$ is plotted in the $x\text{−}z$ plane in terms of the harmonic frequency ${\omega }_{x0}=760$ Hz (the calculated value from the trapping potential). The radial and axial atom densities $n\left(x\right)$ and $n\left(z\right)$ are plotted in the left and bottom frames for a Fermi gas of $1.6\times {10}^5$ atoms per spin state at $T/T_F=0.6$ using 1D Thomas-Fermi distribution [11]. The red dashed lines show the positions of the Fermi radii in the radial (${\sigma }_x$) and axial (${\sigma }_z$) directions, where the local trapping frequency drops to ${\omega }_x({\sigma }_x,0,0)=0.89{\omega }_{x0}$ and ${\omega }_x(0,0,{\sigma }_z)=0.55{\omega }_{x0}$.

Figure 2

The dependence of parametric excitation on the modulation frequency. The radial and axial NIMS, the normalized atom number, and the radial and axial energies are shown from the top to the bottom. All the data have a modulation time $t_m=500$ ms and a modulation amplitude $\delta =0.15$. The harmonic frequency ${\omega }_{x0}=740$ Hz is the measured value. The dashed lines indicate the average value without parametric modulation ($\delta =0)$. The solid lines show the simulation results.

Figure 3

The dependence of the radial and axial energies on the modulation amplitudes. (a) The absorption images of the atom clouds show a dramatic decrease of the axial cloud sizes with an increase of modulation amplitudes, where ${\omega }_m=1.5{\omega }_{x0}$. (b) The dependence of the radial energies on the modulation amplitude. (c) The dependence of the radial energies on the modulation amplitude. In both (b) and (c), blue triangles are at the modulation frequency $1.5{\omega }_{x0}$, and red squares are at $2.0{\omega }_{x0}$. The solid lines represent the simulation of the anharmonic oscillator model using the same simulation parameters for the frequency dependence.